Plasma processing apparatus are widely used for coating, etching or otherwise treating a variety of materials. Such apparatus are widely used in the manufacture of semiconductor devices and have particular applicability for the deposition of thin film electronic materials.
In a typical plasma process, electromagnetic energy is absorbed by a gas which is usually maintained at subatmospheric pressure. The electromagnetic energy ionizes the gas and creates a plasma therefrom. The plasma provides a highly reactive chemical environment which may be utilized to etch, oxidize or nitride a workpiece; or to deposit a variety of materials. Microwave energized plasma processes are being increasingly employed because the highly reactive environment created by the microwaves provides for the rapid and economical processing of a variety of materials. Within the context of this application "microwave energized plasma processing apparatus" shall include all systems in which microwave energy generates a plasma which is employed to treat a workpiece, deposit a material or otherwise facilitate a chemical reaction.
Ignition (also termed initiation) of the plasma is particularly important in microwave energized processes. A body of unionized gas disposed in the plasma apparatus is electrically insulating and in most instances will be a very poor absorber of microwave energy; in contrast, an ionized body of gas is electrically conductive and is a very good absorber of microwave energy. As a result, initiation of the plasma is typically difficult and requires very high levels of microwave power relative to the level of power required to sustain the plasma. This situation can present significant problems since the chamber with the unionized gas represents a non-resonant, reactive load and as such can reflect a great deal of the microwave power back to the microwave source thus damaging it. Additionally, the change in impedance once plasma is initiated will require an adjustment of power level, thereby complicating the control system of the apparatus.
The problem of plasma ignition in microwave energized apparatus has been recognized in the art and various approaches thereto have been implemented. As noted above, the level of power applied to the system may be switched from a high, ignition level to a lower operating level; although, as noted, this approach does not solve problems of reflected power and requires complicated control circuitry. Another approach to the problem is to pre-ionize the atmosphere in the plasma chamber, and as noted in U.S. Pat. No. 4,888,088, ionization may be initiated by use of an auxiliary set of radio frequency energized electrodes. As disclosed in U.S. Pat. No. 4,609,808, a high voltage spark may be employed to begin ionization of the gas. A similar approach is presented in U.S. Pat. No. 4,965,540 wherein a high voltage, high frequency tesla coil is coupled to an igniter wire which is disposed in the plasma chamber. Another approach has been to use ultraviolet light for purposes of initiating ionization and U.S. Pat. No. 4,673,456 discloses one such system.
It has been found that the power level for ignition of a microwave energized plasma is to some degree proportional to the pressure of the gas in the chamber, and ignition of plasma is generally easier at a higher pressure than at a lower pressure. U.S. Pat. No. 4,507,588 discloses a microwave plasma system wherein plasma ignition takes place at a pressure of approximately 300 microns while operation of the system occurs at a substantially lower pressure of 10.sup.-4 torr.
While various systems and methodologies have been implemented in the prior art to ignite a microwave energized plasma, it will be appreciated that the prior art approaches all necessitate additional steps in the operating cycle of the equipment and/or complicated hardware. For example, ionization must be separately initiated by activating spark coils, lamps, auxiliary electrodes and the like; or, power levels and/or pressure levels must be varied during the start-up of the equipment. Clearly, there is a need for a means for initiating a microwave plasma which does not necessitate significant changes in the operational parameters of the equipment and which avoids the need for incorporating complex auxiliary hardware into the system.
The present invention provides an igniter for a microwave energized plasma apparatus, which need not be separately controlled, either during the start-up or operational phase of the plasma equipment. The igniter of the present invention is simple in operation and may be readily retrofitted into present plasma systems. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.